1. Field of the Invention
The present invention relates to a device which will automatically admit a gas as needed into the space above the liquid level of a sealed storage vessel. More specifically the device is a valve and generally inert gases are admitted into low pressure liquid storage vessels to blanket the liquid. A tank blanketing valve is a device which will automatically admit a gas, usually inert (such as nitrogen), as needed into the space above the liquid level of a sealed low pressure liquid storage vessel. Many storage tanks are sealed because the liquid contents often emit hazardous vapors into the atmosphere. One reason for a blanket is to prevent a vacuum and collapse of the tank when a vacuum tends to form due to one of several reasons; cooling of the tank walls at nights or during a sudden rain on a hot day, and/or when the liquid contents are being withdrawn. Another reason for a blanket of inert gas is to maintain product integrity by keeping out air, which contains moisture and some elements that could cause an undesirable chemical reaction with the stored contents.
2. Related Art
Gas blanketing is the injection of a protective gas blanket into the vapor space in liquid storage tanks. Typical locations of application are petroleum refineries, storage depots, and chemical plants.
The purpose of gas blanketing is to maintain an artificial atmosphere above the liquid in storage to prevent the entrance of air which, due to its oxygen content, can cause corrosion of the tank or react with the liquid and to prevent the entrance of atmospheric moisture with its resultant condensation. The product in storage will dictate the gas used, the most common being oxygen free nitrogen. The usual pressures employed are very low-being on the order of less than 1" water column (WC). Typical pressures are 0.5" WC. The pressure is held at all times including constant liquid level or during liquid withdrawal. During liquid withdrawals the volume removed is replaced with the inert gas to maintain a constant pressure and complete blanketing. Likewise with sudden drops in tank pressure due to atmospheric conditions such as sudden rain shower the inert gas is added.
Most existing blanketing valves are regulators modified for tank blanketing use. For example, U.S. Pat. No. 4,274,440 describes such a system using such regulators. A specific device used for this purpose for many years is described in U.S. Pat. No. 3,529,622. Again this particular valve is a pressure regulator adapted for the blanketing use.
It is an advantage of the present invention that a new valve is provided which is specifically designed for the purpose of gas blanketing. A further advantage is a minimum of piping. It is a feature of the present invention that the valve opens and closes as required to maintain a predetermined blanket pressure. A further feature is that the valve is pilot operated to minimize the pressure spread between lock-up and full open, thereby maintaining a closely controlled blanket pressure. A further advantage is the elimination of an expensive molded elastomeric diaphragm and the need to replace such diaphragm.
A particular advantage of the present invention is the replacement of extensive regulator systems with a single device. These and other advantages and features will become apparent from the following.